Laterally π‑Extended Dithia[6]helicenes with Heptagons: Saddle-Helix Hybrid Molecules

A laterally π-extended
dithia[6]­helicene 1,
representing an interesting saddle-helix hybrid molecule containing
an unusual heptagon, has been synthesized by MoCl5-mediated
oxidative stitching of tetrakis­(thienylphenyl)­naphthalene precursor 2 involving reactive-site capping by chlorination and subsequent
Pd-mediated dechlorination of tetrachlorinated intermediate 1-Cl4. Highly distorted, wide helical
structures of dithia[6]­helicenes (1 and 1-Cl4) were clarified by single-crystal X-ray
diffraction analyses where heterochiral slipped π–π
stacking was displayed in a one-dimensional fashion. Notably, theoretical
studies on the thermodynamic behavior of 1 predicted
an extraordinarily high isomerization barrier of 49.7 kcal·mol–1, which enabled optical resolution and chiroptical
measurements. Electronic structures of these huge helicenes were also
examined by photophysical and electrochemical measurements.